Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator
Net Income	$ 236.0	$ 203.8	$ 683.4	$ 641.0
Denominator (000s)
Weighted Average Common Shares - Basic	233,752,000	245,400,000	236,744,300	248,372,900
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	461,500	924,400	399,500	855,200
Weighted Average Common Shares - Assuming Dilution	234,213,500	246,324,400	237,143,800	249,228,100
Net Income Per Common Share
Basic	$ 1.01	$ 0.83	$ 2.89	$ 2.58
Assuming Dilution	$ 1.01	$ 0.83	$ 2.88	$ 2.57